The third category is when they are posing while licking their own boobs, or their own nipples, if their tits are big enough for them to manage that.
I guess they are already quite horny when they are doing that, or they are just trying to get the photographer horny!!!!
Whatever the reason, needless to say that those are my favorite photos of the topic.
And for those of you who didn't get it quite yet, here is the sample: